Acousto-Optic Tuning of Lasers

ABSTRACT

A semiconductor laser tuned with an acousto-optic modulator. The acousto-optic modulator may generate standing waves or traveling waves. When traveling waves are used, a second acousto-optic modulator may be used in a reverse orientation to cancel out a chirp created in the first acousto-optic modulator. The acousto-optic modulator may be used with standing-wave laser resonators or ring lasers.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is a continuation of U.S. patent applicationSer. No. 16/298,873, filed Mar. 11, 2019 , titled “Acousto-Optic Tuningof Lasers” (now U.S. Pat. No. 10,615,562), which is acontinuation-in-part of U.S. patent application Ser. No. 14/636,058,filed Mar. 2, 2015, titled “Acousto-Optic Tuning of Lasers” (now U.S.Pat. No. 10,230,210), which claims the benefit of U.S. ProvisionalPatent Application Ser. No. 61/947,067, entitled “Acousto-Optic Tuningof QCLs and ICLs,” filed Mar. 3, 2014, which applications areincorporated in their entirety here by this reference.

TECHNICAL FIELD

This invention relates to tuning quantum cascade lasers and inter-bandcascade lasers.

BACKGROUND

Quantum cascade lasers (QCL) are lasers in which the gain spectrum istypically broader than approximately 5% of the central wavelength of thelaser. In typical configurations (such as Fabry-Perot configurations,exemplified by FIG. 2 without the grating 14), i.e., with one facet ofthe QCL 10 with high reflectivity coating and the exit facet withcontrolled reflectivity anti-reflection coating 12, the QCLs 10 willproduce very high power output, approaching power greater than 4 W at awavelength of approximately 4.6 micrometer. However, as shown in FIG. 1,this output occurs in a bandwidth of approximately 250 nm around thecentral wavelength.

Such broadband operation is acceptable when the precise wavelength orthe bandwidth of the output is not critical, for example, fordirectional infrared countermeasures (DIRCM) targeting, beacon, andillumination applications. On the other hand, there are a significantnumber of very important applications, where the laser output must benarrow band, for example, less than 1 nm wide, and tunable over somewavelength region. These applications include spectroscopy and sensorsfor detection of pollutants, chemical warfare agents, toxic gases andexplosives. To obtain a “single frequency” output from a broadband gainspectrum laser such as that from a QCL, a wavelength dispersive elementneeds to be introduced within the laser cavity so that only one selectedwavelength can resonate. Such dispersive elements include diffractiongratings 14 (FIGS. 2 and 3), prisms 18 (FIG. 4) and tunable or otherwisenarrow band filters 20 (FIG. 5).

A key feature of all of these schemes is that mechanical motion isrequired to tune, i.e., change, the wavelength of the laser since thewavelength selection is dependent on the angle as shown in the FIGS.2-5. All the techniques shown in these figures permit tuning of thelaser wavelength over the entire gain spectrum (as long as the roundtrip optical gain exceeds total cavity losses). But, the tuning is slowbecause of the mechanical motion of a discrete, dispersive element(grating, prism or filter) and not appropriate for applications callingfor ruggedness, such as for sensors that would be deployed in the field,carried by personnel or mounted on vehicles. There are many applicationsthat require very rapid tuning because there is a need to obtain acomplete spectrum of the object under examination in a very short time.Such applications include studies of time dependent combustion dynamicsand explosion dynamics, time dependent spectral changes during chemicaland biological reactions, rapid examination of an improvised explosivedevice (IED) in standoff detection mode and tracking the release oftoxic gases.

There is yet another way of obtaining narrow linewidth output from anotherwise broadband QCL. This is the use of distributed feedbackgrating, which is embedded within the gain structure of the laser. Suchlasers are useful because they are simple to fabricate and are rugged.However, tunability is quite limited around the design wavelength of thedistributed Bragg grating. Typical tuning range for distributed feedbacklasers (DFB) is limited to approximately 5 cm⁻¹ around the designwavelength of the grating. This is but a small fraction of the gainspectrum width of the QCL. The tuning can be carried out either byvarying the QCL drive current or by changing the temperature of the QCL.In either case, no mechanical motion is required. The thermal tuning isslow while the electrical current driven tuning can be relatively fast.However, for obtaining broadband tuning, DFB lasers are inappropriate.

For the foregoing reasons there is a need for rugged, rapid broadbandtuning of quantum cascade lasers.

SUMMARY

The present invention permits rapid broadband tuning of semiconductorlasers, such as quantum cascade lasers and interband cascade lasers,electronically without the use of any mechanical lotion for thewavelength selection, by utilizing an acousto-optic modulator (alsocalled acousto-optic filter), thereby improving the ruggedness of thelaser. The acousto-optic modulator ay generate traveling waves orstanding waves. When using traveling waves, a second acousto-opticmodulator may be used in the reverse orientation compared to the firstacousto-optic modulator to cancel out any Doppler frequency shifts fromthe first acousto-optic modulator. The acousto-optic modulator can beused with standing-wave laser resonators or ring lasers.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph of a spectral output from a MWIR high power quantumcascade laser in Fabry-Perot configuration (no internal wavelengthdispersion component).

FIG. 2 shows a schematic of a diffraction grating tuned broadband laser(Littrow configuration).

FIGS. 3 shows a schematic of another diffraction grating tuned broadbandlaser (Littman-Metcalf configuration).

FIG. 4 shows a schematic of a prism tuned broadband laser.

FIG. 5 shows a schematic of a narrow bandwidth filter tuned broadbandlaser.

FIG. 6 shows a schematic of an embodiment of an acousto-optic modulator(filter) tuned quantum cascade laser.

FIG. 7 shows a schematic of another embodiment of an acousto-opticmodulator (filter) tuned quantum cascade laser.

FIG. 8 shows a schematic of an embodiment of a standing waveacousto-optic modulator tuned quantum cascade laser.

FIG. 9 shows a schematic of another embodiment of a standing waveacousto-optic modulator tuned quantum cascade laser with optical outputthrough zero order diffraction.

FIG. 10 is a graph showing discreet laser wavelength tuning as afunction of acoustic driving frequency for a 1 cm long acousto-opticmodulator.

FIG. 11 is a graph showing discreet laser wavelength tuning as afunction of acoustic driving frequency for a 2 cm long acousto-opticmodulator.

FIG. 12 shows a schematic of an embodiment of a traveling waveacousto-optic modulator tuned quantum cascade laser (Littrowconfiguration).

FIG. 13 shows a graph of a tuning curve for the acousto-opticmodulator-controlled external cavity quantum cascade laser (EC QCL)setup.

FIGS. 14A-C show measured output spectra for the same QCL outputwavelength at three different acousto-optic modulator frequencies.

FIG. 14D shows measured output spectra from FIGS. 14A-C overlapped ontoa single graph for comparison.

FIG. 15 shows a graph of emission spectrum linewidth dependence onacousto-optic modulator central frequency.

FIGS. 16A-B show switching time data for the acousto-opticmodulator-controlled EC QCL setup.

FIG. 17 shows a schematic defining the beam size D_(B) and sound wavepropagation length to the beam L.

FIG. 18 shows a graph of simulated vs. experimental data for IPA(isopropyl alcohol) absorption.

FIG. 19 shows a schematic of an embodiment of a ring laser utilizing anacousto-optic modulator.

FIG. 20 shows a schematic of another embodiment of a ring laserutilizing an acousto-optic modulator.

FIG. 21 shows a schematic of another embodiment of a ring laserutilizing an acousto-optic modulator.

DETAILED DESCRIPTION OF THE INVENTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of presently-preferred embodimentsof the invention and is not intended to represent the only forms inwhich the present invention may be constructed or utilized. Thedescription sets forth the functions and the sequence of steps forconstructing and operating the invention in connection with theillustrated embodiments. It is to be understood, however, that the sameor equivalent functions and sequences may be accomplished by differentembodiments that are also intended to be encompassed within the spiritand scope of the invention.

Emission wavelength of a single Fabry-Perot (FP) QCL chip with ananti-reflection-coated back facet is controlled with a dispersiveelement located outside the laser medium 102, i.e. using the externalcavity (EC) approach. However, in contrast to traditional externalcavity QCLs with a moving grating for example, the rapid wavelengthtuning in the present invention is achieved using an electricallycontrolled acousto-optic modulator (AOM) 106. Compared with the DFBconfiguration, the present invention provides continuous tunability ofthe lasing wavelength over the entire gain bandwidth of the QCL; andcompared with EC QCLs with a grating, which do provide continuous andbroad tunability, the present invention has no moving parts.

An AOM 106 comprises a transparent material 107 having a piezoelectrictransducer 112 attached at one end, and an acoustic absorber 114attached at the opposite end. The piezoelectric transducer 112 creates asound wave that is propagated through the transparent material 107towards the acoustic absorber 114. In particular, high-frequencyacoustic wave in AOMs 106 may be generated in a transparent material 107(germanium in case of long wavelength infrared (LWIR) region) and thisacoustic wave forms an index grating. The AOM 106 also has two opposingfacets adjacent to each end of the opposing ends of the AOM 106. The twofacets may have anti-reflection coatings 117 at the optical wavelengthof interest on the two facets through which the laser radiation (i.e.beam) passes.

All commercially available AOMs, have anti-reflection coatings 117 atthe optical wavelengths of interest on the two facets through which thelaser radiation passes.

In some embodiments, two AOMs 106, 108, with opposed travelling acousticwaves 115 a, 115 b, respectively, may be used so that the Dopplerfrequency shift in the optical wave introduced by the first AOM 106 iscancelled by the complementary Doppler shift introduced by the secondAOM 108. In other words, the second AOM 108 comprises a transparentmaterial 109 having a first end and a second end, a second piezoelectrictransducer 116 at the first end and a second acoustic absorber 118 atthe second end, wherein the orientation of the second AOM 108 isreversed compared to the first AOM 106 as shown in FIGS. 6 and 7. In anyembodiment in which a Doppler shift in the optical wave is anticipated,for example, when using travelling waves in the AOM, a second AOM 108may be used in the opposite orientation as the first AOM 106 to counterthe Doppler shift.

The geometry using two AOMs 106, 108 permits continuous tuning of theQCL (as opposed to discrete tuning in steps). The incident light wavesemitted by the laser are deflected by the travelling acoustic gratingscreated in the AOMs. The angle of the deflection is controlled by thechoices of optical and acoustic wavelengths according to equation 1below:

$\begin{matrix}{{\sin \theta_{B}} = \frac{\lambda_{i} \cdot v_{a}}{2nV_{a}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

where θ_(B) is the Bragg diffraction angle, λ₀ is the free space opticalwavelength, n and V_(a) are the refractive index of and the acousticvelocity in the AO material, respectively, and v_(a) is acousticfrequency. The direction of the lasing is determined by the laserresonator cavity so the output laser wavelength can be electronicallyselected by changing the acoustic frequency of the AOM 106, thuschanging the Bragg angle condition for the selected wavelength.Deflection efficiency of approximately 90 percent, comparable to that oftraditional diffraction gratings, has been demonstrated for LWIR AOMs.The response time of the modulators is determined by the transit time ofthe acoustic wave across the material 107 and is consistent with highspeed (rapid) measurements. Notice that once the system is aligned,there is no mechanical motion involved for tuning the wavelength of theoutput. The AOMs 106, 108 are aligned within the cavity and tuning ofthe output wavelengths is achieved by changing the driving acousticfrequency of the modulators according to Equation 1.

The deflection efficiency depends on modulator radiofrequency (RF) powergenerated by the radiofrequency generators 125, 126. The higher the RFpower, the higher the efficiency, leading to higher feedback strength.

Two possible systems geometries, the one shown in FIG. 6 (correspondingto Littrow configuration) and another one similar to the Littman-Metcalfgeometry for traditional EC QCLs (FIG. 7) are possible, among others. Anembodiment of the broadly tunable quantum cascade laser is shown in FIG.6. The laser comprises a gain medium 102, at least one collimating lens104 a, b, at least one acousto-optic (AO) modulator 106, 108 and atleast one highly reflective mirror 110.

In the Littman-Metcalf configuration, the output beam 124 exits from themodule through the zero order diffraction of the grating created by theAOM 106. The configuration in FIG. 7 may have some advantage over thatin FIG. 6. Using the undiffracted beam 120 (or the zero orderdiffraction beam in the traditional grating nomenclature) has theadvantage of using one fewer collimating lens 104 b and associatedadvantage of reduced effort for aligning two lenses. The removal of onelens also reduces the overall cavity losses, thus will lead to a broadertuning range for the same QCL, chip. Therefore, as shown n FIG. 7, acollimating lens 104 b at one end can be replaced with a high reflectivecoating 130 at the gain media 102 of the QCL at that end.

Another variation of the configurations shown in FIGS. 6 and 7 ispossible if continuous tuning is not required as may be the case whenthe tunable laser output interacts with a broad linewidth absorber, suchas with chemical warfare agents, explosives vapors, and large moleculetoxic industrial chemicals. This alternate variation, shown in FIGS. 8and 9 correspond to the two configurations in FIGS. 6 and 7,respectively. The primary difference arises from the use of a standingacoustic wave 113 (as opposed to the traveling acoustic waves 115 a, b,in FIGS. 6 and 7). For example, the acoustic modulator 106 may not havean acoustic absorber 114 at one end. The diffraction of the laser beamfrom the standing acoustic wave is not accompanied by a Dopplerfrequency shift in the optical wave and thus a single resonant AOM 106can be used, leading to substantial simplification of the optical setupand reduction in cost, as well.

As mentioned above, this configuration does not permit continuouswavelength tuning of the QCL because the standing wave nature of theacoustic grating requires an acoustic resonance in the modulator andthus only discrete acoustic wave frequencies are permitted, determinedby the length of the AOM element. The acoustic resonance condition iswritten as shown in Equation 2 below:

Nλ _(ac)=2L   Equation 2

where N is the number of acoustic resonances in the AOM 106, λ_(ac) isthe acoustic wavelength and L is the length of the AOM 106. Thus thediscrete permissible acoustic frequencies, v_(ac), for diffraction ofthe light beam are given by Equation 3 below:

$\begin{matrix}{v_{ac} = \frac{N\nu_{ac}}{2L}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

where v_(ac) is the acoustic velocity in the AO material 107. FIGS. 10and 11 show calculated discrete frequencies that are possible in a GeAOM of 1 cm and 2 cm lengths, respectively.

Thus, for situations where continuous tuning not required, the standingwave AOM can provide an alternate and simpler solution since even for a1 cm long AOM, over 100 discrete wavelengths can be obtained betweentunable wavelengths of 7 μm and 11 μm.

Overall, the invention has several important advantages, over thegrating, prism or filter based EC QCL, for practical field applications.For example, the tuning speed is improved. Typical RF modulationbandwidth for commercially available AOMs is in the range of tens ofMHz. Therefore, the required switching time between arbitrarywavelengths of under 1 microsecond can easily be achieved. Thisswitching time is at least five orders of magnitude faster than that forEC QCLs with a grating.

The flexibility of the wavelength control with AOMs also offers aninteresting option of multi wavelengths operation when AOM wave iscomposed of several discrete frequencies. Each of the sound wavefrequencies in this case will force the laser to operate at thecorresponding optical frequencies. The multi wavelength operation can beimportant in spectroscopy when several spectral lines need to be trackedat the same time.

Also, the ruggedness is improved. In contrast to EC QCLs with a grating,wavelength tuning mechanism does not require any mechanical motion. Themodule can therefore be ruggedized to meet the most stringentrequirements for field applications.

In addition, the yield (cost) is improved. The broadly tunable quantumcascade laser comprises a single QCL chip. The standard single-QCLfabrication process has a relatively high yield, exceeding 50 percentfor watt-level LWIR devices. In case of QCL arrays, yield, however,quickly drops with increase in number of elements. For comparison, evena 10-element array processed from the same material would have a yieldof less than 1 percent. Number of elements in a DFB QCL array requiredto cover the 7 to 11 micrometer tuning region can be as high as onehundred. As a consequence, DFB QCL arrays are projected to have a verylow yield, making this technical approach impractical.

With regards to epi-down mounting, the present invention is compatiblewith QCL epi-down mounting that is now commonly employed to lower activeregion temperature of high average power devices. QCL arrays, on theother hand, are typically mounted epi-up to preserve individualelectrical control for each of the emitters in the array. Overheating ofthe epi-up mounted array will make the goal of reaching high averageoptical power significantly more difficult, or even impossible.

With regards to reliability, all of the components in the proposedmodule have been in commercial use for some time and have proven theirlong-term reliability. The high power FP QCLs have been in commercialproduction for nearly a decade and the AOMs have been commerciallydeployed for several decades. Thus, the two critical components in theproposed configuration, namely FP QCLs and AOMs, represent maturetechnologies.

AOMs have been successfully used for rapid and random access discreettuning of CO₂ lasers on their 9.6 micrometer and 10.6 micrometer laserbands and continuous tuning of Ti:sapphire lasers. Research alsoprovides earlier data on tuning of dye lasers using acousto-opticmodulators. However, because of the only recent development of QCLs,acousto-optic tuning of QCLs has not been attempted.

This technology may be applicable to the interband cascade lasers aswell.

EXAMPLES

Reported is the first operation of tunable external cavity quantumcascade lasers with emission wavelength controlled by an AOM. Along-wave infrared quantum cascade laser wavelength is tuned fromapproximately 8.5 micrometer to approximately 9.8 micrometer when theAOM frequency is changed from 41 MHz to 49 MHz. The laser delivered over350 mW of average power in the center of the tuning curve in a linewidthof 4.7 cm⁻¹. Measured wavelength switching time from the edge of thetuning curve to the center of the tuning curve is less than 1microsecond. Initial spectral measurements of infrared absorptionfeatures of gaseous isopropyl alcohol were carried out, whichdemonstrate a capability of obtaining complete spectral data fromapproximately 8.5 micrometer to approximately 9.8 micrometer in lessthan 20 microseconds. The demonstration paves a way for a new generationof tunable QCLs providing ruggedness, fast tuning and high powercapability in the infrared spectral region.

The AOM 106 tuned QCL configuration is shown in FIG. 12. Thepiezoelectric transducer 112 of the AOM creates a traveling acousticwave 115 in the germanium crystal transparent medium 107. The soundwave, in turn, produces a spatial periodic variation in refractive indexof the crystal, i.e. it creates a phase grating. The undiffractedoptical beam 120 incident on the AOM 106 experiences a wavelengthdependent deflection. The diffracted beam 122 subsequently reflects backfrom the mirror 110 to the AOM 106 and couples back into the gain medium102, completing the wavelength-dependent feedback loop. The outputoptical beam 124 from the gain medium 102 emerges from the other side105 (distal facet) of the laser.

The period of the acoustic grating is electronically controlled, whichallows for the rapid change of the diffracted wavelength. Typicalmodulation bandwidth for commercially available AOMs is in the range oftens of MHz. Hence, the required switching time, between arbitrarywavelengths, of under 1 microsecond can be achieved. This switching timeis at least five orders of magnitude faster than that for EC QCLs with agrating. In addition, in contrast to EC QCLs with a grating, wavelengthtuning mechanism does not require any mechanical motion. The module cantherefore be ruggedized to meet the most stringent requirements forfield applications.

The frequency of the AOM used in this example was adjustable in therange from 35 MHz to 55 MHz. FIG. 13 shows a measured tuning curve for a9 micrometer QCL, with the QCL design described in A. Lyakh R. Maulini,A, Tsekoun, R. Go, and C. K, N. Patel, “Multiwatt long wavelengthquantum cascade lasers based on high strain composition with 70%injection efficiency”, Opt. Expr. 22, 24272 (2012) (which isincorporated here by this reference), operating in a quasi-CW mode (350nanosecond pulses and 50 percent duty cycle) as AOM frequency waschanged from 41.7 MHz to 48.5 MHz (approximately ⅓ of the availablefrequency range). AOM input power was fixed at 35 W during the testing.The emission. wavelength for the EC laser tuned from 1020 cm⁻¹ (9.8micrometer) to 1170 cm⁻¹ (8.5 micrometer). The tuning range availablefrom a single laser can be increased using broader gain QCL chips.

Measured QCL emission spectrum linewidth for AOM central frequency of 45MHz was 4.7 cm⁻¹. The linewidth can be changed by aligning the system sothat it operates at another AOM central frequency (different Braggcondition). Employment of a higher AOM frequency will result into alarger number of illuminated acoustic grating periods and, therefore, toa proportionately narrower width. This effect is illustrated in FIGS. 14and 15. The increase in the central AOM frequency from 35 MHz to 55 MHzleads to a reduction in emission linewidth from 5.7 cm⁻¹ to 3.9 cm⁻¹.

As shown in FIG. 17, the response time of the change in the opticalwavelength with a change in the AOM frequency has two components: (1)propagation time of the acoustic wave from the acoustic transducer 112to the edge of the optical beam L going through the AOM, and (2) thepropagation time for the acoustic wave across the optical beam D_(B).The first “delay” is the latency time and does not represent theresponse time of the change the optical wavelength when AOM frequency ischanged. The latency time can be shortened, almost arbitrarily, byreducing the distance between the acoustic transducer and the positionof the optical beam. The actual response time, therefore, is determinedby the acoustic wave transit time across the optical beam. For thepresent case, the time it takes an acoustic wave to propagate from thepiezoelectric transducer across the germanium material to the edge ofthe optical beam t₁ and time for the acoustic wave to cross the opticalbeam is T₂. The time t₂ is the actual response time of the AOM forchanging the optical wavelength. As mentioned above, t₁ can be shortenedto almost zero. However, there are limitations on how short t₂ can be.If t₂ is made too short by making the optical beam diameter small, theoptical wave will interact with a fewer number of acoustic waves andtherefore the linewidth of the output will increase. The linewidth ofthe optical output can be reduced by making the optical beam diameterlarger, but that occurs at the expense of the response time. The optimallinewidth/response time balance is application driven.

FIGS. 16A and 16B show measured response time of the system as AOMfrequency switches from 35 MHz (outside of the tuning curve) to 45 MHz(center of the tuning curve). FIG. 14B shows the AOM frequency controlsignal and FIG. 14A shows the optical signal from the laser, whichconsists of pulses of duration 350 nanosecond pulses with a pulserepetition period of approximately 700 nanoseconds.

AOM frequency is initially equal to 35 MHz, outside of the tuning curveof the laser. The optical signal from the laser is at zero level sincethe Bragg condition is not satisfied anywhere in the laser gainspectrum. AOM frequency is abruptly changed to 45 MHz at time t=0. AOMfrequency of 45 MHz corresponds to the peak of the gain curve. It takesapproximately t₁=L/v_(s)=1.25 microseconds for the acoustic signal withthe new frequency to reach the area where the optical beam is incidenton the crystal (v_(s) is sound velocity and L is defined in FIG. 17).When the sound signal with the new frequency reaches that area, theoptical signal starts growing. It reaches its maximum value at t₂=550nanoseconds+t₁, in a good agreement with the calculated travelling timeacross the beam area t₂=D_(B)/v_(s)=3.4 millimeter/(5.5millimeter/microsecond) approximately 600 nanoseconds (FIG. 16A). TheAOM frequency switches back to 35 MHz at t=3 microseconds. The processis repeated: it takes about t₁+t₂=1.8 microseconds for the laser tocompletely shut down (FIG. 16B).

As mentioned above, the latency time t₁ is proportional to L and can bealmost arbitrarily minimized by designing the AOM so that thepiezoelectric transducer generating the sound wave is positioned closerto the beam area (L in FIG. 17 as short as possible). Time t₂, on theother hand, is fixed for the same choice of material and beam size. Itis, however, already well below 1 microsecond. Thus, if the AOMfrequency was changed between two values, for example, f₁ and f₂, thelaser wavelength will switch between two corresponding values in a timeless than 1 microsecond, the latency time notwithstanding.

To demonstrate the rapid spectral measurement capability for theAOM-controlled EC QCLs, a transmission absorption spectrum of isopropylalcohol vapor that has a broad infrared absorption spectrum. Themeasurement was done in the sweep mode when acousto-optic frequency wasvaried from 42 MHz to 48 MHz. FIG. 18 shows the comparison between anIPA spectrum generated using the HITRAN database and collectedexperimental data. The experimental data were captured in a single AOMfrequency sweep from 42 MHz to 48 MHz that took less than 20microseconds.

In conclusion, the first data for AOM-controlled external cavity QCLs ispresented. These devices offer the advantage of very fast tuningcapability with spectral measurement me of under 20 seconds. Theconfiguration does not involve any moving parts and therefore can beruggedized for demanding field applications.

The present invention may also be applied to ring laser geometry asshown in FIGS. 19-21. The ring laser configuration has been extensivelystudied in the near infrared lasers. Compared to traditional standingwave cavity lasers, described in other parts of this patent application,ring cavity lasers can operate in either of the counter propagatingdirections or both. This capability offers advantages including thecapability of unidirectional laser oscillation with at least thefollowing advantages: (1) ring laser operating in a unidirectional mode,eliminates the spatial hole burning effect present in traditional cavitylasers and extracts power from the gain medium uniformly, making ithomogeneous, which increases mode competition between adjacentlongitudinal modes and affords the possibility of single mode operationeven at high power outputs; (2) uniform extraction of power withoutspatial hole burning results in higher single mode power output comparedto that possible with standing wave configuration; (3) ring cavityoperation is less sensitive to small misalignments; and (4) ring cavitylaser operated in a unidirectional mode can be less sensitive to opticalfeedback from external elements.

As shown in FIGS. 19 and 20, the ring laser geometry may comprise thegain medium 102 of the laser having anti-reflection coatings 132 a, b atopposite ends of the gain medium 102, a pair of collimating lenses 104a, b at opposite ends of the gain medium 102, an AOM 106 powered by aradiofrequency generator 125, and a plurality of highly reflectivemirrors 110 a-c. The highly reflective mirrors are positioned so thatone mirror 110 a receives the diffracted beam 122 from the AOM 106 anddeflects the diffracted beam 122 to the second mirror 110 b positionedon the opposite side of the gain medium 102, which in turn deflects thediffracted beam 122 to the third mirror 110 c, which deflects thediffracted beam 122 through one of the collimating mirrors 104 b andback to the gain medium 102. In some embodiments, the AOM 106 may havean acoustic absorber 114 opposite the piezoelectric transducer 112 so asto generate a traveling acoustic wave 115 through the material 107 (FIG.19). In other embodiments, the AOM 106 may not have the acousticabsorber so as to create a standing wave 113 (FIG. 20).

In some embodiments, as shown in FIG. 21, the laser may have a highreflective coating 130 at one end or facet 105 (the distal end) of thegain medium 102 and only one collimating lens 104 at the opposite end orfacet 103 (the near end) of the gain medium 102. The beam is emittedfrom one end or facet 103 of the gain medium 102, through thecollimating lens 104, and passes through the AOM 106. The beam isdiffracted through the AOM 106 and is directed to the first mirror 110a. Through a series of reflections off of the other mirrors 110 b, 110c, the diffracted beam 122 returns to the laser at the opposite end orfacet 105 of the gain medium 102 having the high reflective coating 130.

In embodiments utilizing travelling waves, a second AOM 108 may be usedin the opposite orientation relative to the first AOM 106, and inbetween the first AOM 106 and the first mirror 110 a, so that thetravelling waves from each AOM 106, 108 travel in opposite directions sothat the Doppler shift in the frequency of the diffracted laser beamcreated in the first AOM 106 is cancelled out by the complimentaryDoppler shift introduced by the second AOM 108.

QCLs or gain medium 102 are fabricated out of multilayer structurescomprising AlInAs/GaInAs with controlled and predetermined fractions ofAl and In in AlInAs and controlled and predetermined fractions of Ga andIn in GaInAs. The refractive index, n, of this multilayer structure endfacet is approximately 3.4. A pristine end facet surface 103, 105 has areflectivity given by:

$\begin{matrix}{R = \left( \frac{n - 1}{n + 1} \right)^{2}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

Where n is approximately 3.4, R is 0.3.

Thus, for a QCL chip with both uncoated facets, the device will lase asa Fabry-Perot cavity formed by the 30% reflection from the end facets103, 105. For Fabry-Perot configuration QCLs with both facets 103, 105uncoated, the laser output will occur from both ends, and the outputwavelength spectrum will be broad, as determined by the gain bandwidthof the QCL, which will be typically 100 cm⁻¹ to 200 cm⁻¹. For suchlasers to be useful devices, generally one facet (e.g. facet 105) iscoated with a totally reflective or high reflectivity coating 130,either multilayer dielectric coating or more generally evaporated goldcoating. For the latter case, a thin layer of AlN or similar highresistivity material, transparent for the wavelengths of interest, isdeposited on the end facet before the gold coating to make sure that thegold coating does not electrically short the QCL. A high reflectivitycoating 130 will reflect substantially all of the laser radiation. Now,the Fabry-Perot cavity QCL with the high reflectivity coating 130 willemit laser radiation only from the uncoated facet (e.g. facet 103).

Lasing operation and the power output from a Fabry-Perot cavity QCL isdetermined by the amount of optical gain produced in the chip (gainmedium 102) and the total optical loss. For a chip (gain medium 102)with one facet high reflectivity (HR) coated 130, the loss consists ofthe inherent loss within the QCL chip and the 70% loss through theuncoated facet (exit facet). Of course, in order for the chip (gainmedium 102) to lase, the gain must exceed the total loss. However, it isrecognized by scientists, that the 30% reflectivity of the uncoated exitfacet is too high to extract optimum amount of power from the lasercavity. Thus, multilayer dielectric coating may be deposited on the exitfacet such that the reflectivity is less than 30%, so that more powercan be extracted. This type of coating is referred to as controlledreflectivity anti-reflection coating or partially reflective coating133. The exact amount of reflectivity will depend on the details of theamount of gain produced by the QCL chip and other losses in the cavity.In the preferred embodiment, controlled reflectivity anti-reflectioncoating 133 is applied such that reflectivity may be from approximately10% to approximately 30%. In some embodiments, the reflectivity may befrom approximately 15% to approximately 25%. The various coatingtechniques, i.e. high reflective coating 130, anti-reflection coating132 a, b, and controlled reflectivity anti-reflection coating 133 can beapplied in any of the embodiments described herein, including, but notlimited to, what is shown in the figures.

In the patent application under consideration, the lasing cavity is notthe Fabry-Perot cavity formed by the end facets. As a matter of fact, itis necessary to suppress Fabry-Perot cavity lasing because tunablewavelength output is desired (as opposed to a broad band output,characteristic of a Fabry-Perot cavity). Thus the lasing cavitycomprises reflection from one of the facets of the QCL chip (e.g. thedistal facet 105) and the reflecting mirror 110 c beyond theacousto-optic modulator 106. The acousto-optic modulator 106 determineswhich specific wavelength is reflected back into the QCL. In order tosuppress Fabry-Perot cavity lasing by the QCL, the near facet (e.g.facet 103) of the QCL chip is anti-reflection (AR) coated 132 a withmultilayer dielectric coatings. Now, in order to extract the maximumamount of tunable power (from the distal end of the QCL chip), the totalgain in the chip and the total losses may be considered. In general, inspite of the losses in the lasing cavity formed by travel through theacousto-optic modulator, reflection from the fixed mirror and thetransmission losses through the exit face, the 30% reflectivity ofnative facet is too high. So, just as with optimizing power output froma Fabry-Perot cavity laser, described above, we deposit a controlledreflectivity anti-reflection coating 133 on the exit facet (e.g. facet105). The reflectivity of this coating is generally less than that of anative facet. Again, the optimum reflectivity of the partiallyreflective anti-reflection coating 133 will depend on the details of thesystem.

In some embodiments, the radiofrequency generator 125, 126 operativelyconnected to the acousto-optic modulator 106, 108 may be configured togenerate two radiofrequencies simultaneously. In some embodiments, oneof the first or second radiofrequencies may be fixed and the other ofthe first or second radiofrequencies may be varied to simultaneouslygenerate two laser wavelengths, one which is fixed and the other whichis tuned. Traditional tunable lasers, using diffraction gratings (FIGS.3 and 4), prisms (FIG. 5) or filters (FIG. 6) are not able to supportsimultaneous operation on two wavelengths, because for any angularsetting of the grating, prism or the filter, only one wavelength canresonate inside the optical cavity.

In some embodiments, one of the first or second radiofrequencies may befixed and the other of the first or second radiofrequencies may bevaried and temporally switched to simultaneously generate two laserwavelengths, one which is fixed in wavelength and unswitched, and theother which is tuned and is temporally switched. Such operation providesthe capability of pumping (exciting) the medium at on wavelength (tunedand switched) and monitoring the response of the medium at the otherwavelength that is tuned but not switched. These measurement techniquesare called “pump-probe” studies.

In some embodiments, one of the first or second radiofrequencies may befixed and temporally switched, and the other of the first or secondradiofrequencies may be varied and temporally switched to simultaneouslygenerate two laser wavelengths, one which is fixed in wavelength andtemporally switched, and the other which is tuned and is temporallyswitched. This arrangement will provide yet another way of carrying outpump-probe studies of a medium in the infrared.

The foregoing description of the preferred embodiment of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention not be limited by this detailed description, but by the claimsand the equivalents to the claims appended hereto.

What is claimed is:
 1. A laser, comprising: a. a quantum cascade lasergain medium having a first end and a second end; b. a first collimatinglens adjacent to the first end of the gain medium and receiving anundiffracted beam from the gain medium, the undiffracted output bearerdefining a first path; c. an anti-reflection coating at the first end ofthe gain medium; d. a first acousto-optic modulator adjacent to thefirst collimating lens and opposite the gain medium and fixed in place,the first acousto-optic modulator receiving the undiffracted beamdirectly from the first collimating lens and subsequently emitting adiffracted beam along a second path, wherein the first acousto-opticmodulator is electronically controlled to produce the diffracted beam ata desired wavelength, the first acousto-optic modulator comprisinggermanium; e. a first reflective mirror positioned to receive thediffracted beam and reflect the diffracted beam back along the secondpath to the first acousto-optic modulator through the first path andback to the gain medium to amplify a wavelength tuned beam; and f. asecond acousto-optic modulator in between the first acousto-opticmodulator and the first reflective mirror, wherein an orientation of thesecond acousto-optic modulator is reversed compared to the firstacousto-optic modulator, and wherein the second acousto-optic modulatoris electronically controlled to produce the diffracted beam at a desiredwavelength, the second acousto-optic modulator comprising germanium. 2.The laser of claim 1, further comprising a reflective coating at thesecond end of the gain medium.
 3. The laser of claim 2, wherein a) thefirst acousto-optic modulator comprises a first end, a second endopposite the first end, and a first piezoelectric transducer at thefirst end of the first acousto-optic modulator; b) the secondacousto-optic modulator comprises a first end, a second end, and asecond piezoelectric transducer at the first end of the secondacousto-optic modulator; c) a first radiofrequency generator forgenerating a first radiofrequency for controlling the firstpiezoelectric transducer; and d) a second radiofrequency generator forgenerating a second radiofrequency for controlling the secondpiezoelectric transducer.
 4. The laser of claim 3, wherein a) the firstacousto-optic modulator comprises a first acoustic absorber at thesecond end of the first acousto-optic modulator; and b) the secondacousto-optic modulator comprises a second acoustic absorber at thesecond end of the second acousto-optic modulator.
 5. The laser of claim4, wherein the laser is an infrared quantum cascade laser or an infraredinterband cascade laser.
 6. The laser of claim 2, wherein the reflectivecoating is selected from the group consisting of a partially reflectivecoating and a highly reflective coating.
 7. The laser of claim 1,further comprising a second anti-reflection coating at the second end ofthe gain medium.
 8. The laser of claim 1, wherein the firstacousto-optic modulator comprises: a) a first end and a second endopposite the first end, b) a first facet adjacent to the first andsecond ends, the first facet having a first anti-reflection coating at adesired optical wavelength, and c) a second facet opposite the firstfacet and adjacent to the first and second ends, the second facet havinga second anti-reflection coating at a desired optical wavelength.
 9. Thelaser of claim 1, wherein a) the first acousto-optic modulator comprisesa first acoustic absorber at the second end of the first acousto-opticmodulator; and b) the second acousto-optic modulator comprises a secondacoustic absorber at the second end of the second acousto-opticmodulator.
 10. The laser of claim 1, further comprising a secondcollimating lens adjacent to the second end of the gain medium.
 11. Thelaser of claim 1, further comprising a plurality of reflective mirrorsto form a ring laser resonator.
 12. The laser of claim 1, wherein thelaser is an infrared quantum cascade laser or an infrared interbandcascade laser.
 13. The laser of claim 1, wherein a) the firstacousto-optic modulator comprises a first end, a second end opposite thefirst end, and a first piezoelectric transducer at the first end of thefirst acousto-optic modulator; b) the second acousto-optic modulatorcomprises a first end, a second end, and a second piezoelectrictransducer at the first end of the second acousto-optic modulator; c) afirst radiofrequency generator for controlling the first piezoelectrictransducer; and d) a second radiofrequency generator for controlling thesecond piezoelectric transducer.
 14. A laser, comprising: a) a quantumcascade laser gain medium having a first end and a second end; b) acollimating lens adjacent to the first end of the gain medium andreceiving an undiffracted output beam from the gain medium, theundiffracted beam defining a first path; c) an anti-reflection coatingat the first end of the gain medium; d) an acousto-optic modulatoradjacent to the first collimating lens and opposite the gain medium, theacousto-optic modulator receiving the undiffracted output beam from thefirst collimating lens and subsequently emitting a diffracted beam alonga second path; e) a reflective mirror positioned to receive thediffracted beam and reflect the diffracted beam back along the secondpath to the first acousto-optic modulator through the first path andback to the gain medium to amplify a wavelength tuned beam; and f) aradiofrequency generator operatively connected to the acousto-opticmodulator, the radiofrequency generator configured to generate a firstradiofrequency and a second radiofrequency simultaneously, wherein thefirst radiofrequency is fixed and is temporally switched, and the secondradiofrequency is varied and is temporally switched to simultaneouslygenerate a first laser wavelength that is fixed in wavelength andtemporally switched, and a second laser wavelength that is tuned and istemporally switched.
 15. The laser of claim 14, further comprising areflective coating at the second end of the gain medium.
 16. The laserof claim 14, wherein the acousto-optic modulator comprises a first end,a second end opposite the first end, a transparent material between thefirst end and the second end of the acousto-optic modulator, and apiezoelectric transducer at the first end of the acousto-opticmodulator.
 17. The laser of claim 16, wherein the acousto-opticmodulator comprises an acoustic absorber at the second end of theacousto-optic modulator.
 18. The laser of claim 16, wherein theacousto-optic modulator comprises: a) a first facet adjacent to thefirst and second ends, the first facet having a first anti-reflectioncoating at a desired optical wavelength, and b) a second facet oppositethe first facet and adjacent to the first and second ends, the secondfacet having a second anti-reflection coating at a desired opticalwavelength.
 19. The laser of claim 14, wherein the laser is an infraredquantum cascade laser or an infrared interband cascade laser.
 20. Thelaser of claim 14, further comprising a plurality of reflective mirrorsto forth a ring laser resonator.